Self-starting arc lamp



Sept. 20, 1966 PAQUETTE ET AL 3,274,427

SELF-STARTING ARC LAMP Filed March 15, 1965 R W E TUL NQL 5 N 8 NE mm YA AH RC V! B Dam-1 Lo -ML- ATTORNEY United States Patent 3,274,427 SELF-STARTING ARC LAMP Raymond E. Paquette, Sunnyvale, and Charles H. Keller,

Los Altos, Calif., assignors to Pek Labs, Inc., Sunnyvale, Calif., a corporation of California Filed Mar. 15, 1965, Ser. No. 439,757 The portion of the term of the patent subsequent to June 14, 1983, has been disclaimed 12 Claims. (Cl. 31434) This invention relates to are lamps and a method for starting the arc, and more particularly to a self-starting arc lamp for one-shot operation.

This is a continuation-inpart of our application Serial No. 258,717, filed February '15, 1963.

As fully explained in the hereinabove referred to pending application, are lamps are in relative common use today in applications requiring very intense sources of light. For example, are lamps are used for oscillographic photography, for solar simulation to determine deterioration due to sunlight, for search lights and the like. To start such arc lamps requires the initial application of a relatively high volt-age starting pulse, which is typically 15,000 volts, and which is supplied by a starting power supply. The high voltage originally required to draw an are between electrodes is necessary for ionizing the gas between the electrodes to cause the arc to flash across the electrodes, the stages being Townsend discharge, glow discharge and are discharge in the'order stated. Once started, the arc is easily maintained by the application of power at a relatively low voltage, typically less than 30 volts, which is provided by a separate sustaining power supply.

For one-shot arc lamps, such as may be used in the optical tracking element of airborne vehicles, the requirement for a starting power supply in addition to the sustaining power supply has been a very serious limitation on their use. Not only must a starting power supply be carried by the airborne system for a single instant of use, but additionally, this power supply must be interconnected with the .sustaining power supply in such a manner that the sustaining power supply and the arc lamp itself will not beburned out by the starting pulse. Further, in case ribbon seals are utilized to form the conductive path through the envelope enclosing the electrodes, means must be provided to carefully limit the starting current to prevent burning out of the ribbon. Typically the energy of such a starting pulse in connection with ribbon arc lamps is limited to 2 watt-seconds.

The above identified co-pending application discloses an arc lamp in which a conductive fusion bridge spans the gap between the electrodes and forms a low resistance electrical path therebetween. The fusion bridge is shaped in such a manner that its cross sectional area progressively increases axially, from a minimum area, in the direction of one or both of the electrodes.

To start the arc lamp therein disclosed, low voltagepower from the sustaining power supply is applied to the electrodes to vaporize this bridge progressively from the minimum cross sectional area outwards toward the electrodes. Until the minimum cross section area is vaporized, there is no gap to bridge. As soon as the minimum cross section area is vaporized a small gap is formed which increases in length until the whole bridge is vaporized and the arc is operating in its normal mode between the two electrodes.

This progressive vaporization of the fusion bridge is analogous to physically withdrawing one of two touching electrodes until proper electrode spacing is achieved for normalyarc operation. While this type of arc lamp is eminent-1y suited for starting the arc, it has been found that for applications requiring survival to extreme environmental conditions, such as very severe shock and vibration, the rigid nature and the various modes of suspension of the fusion bridge did limit its survival capability somewhat. It was also found that a progressively increasing cross section area fusion bridge added to the manufacturing cost of the arc lamp because of support and alignment requirements during the assembly operations.

It is therefore a primary object of this invention to provide an improved self-starting arc lamp for one-shot operation.

It is another object of this invention to provide a selfstarting bridgewire arc lamp which is rugged in construc tion and capable of surviving severe shock and vibration.

It is a further object of this invention to provide a selfstarting one-shot bridgewire arc lamp which is inexpensive to manufacture and reliable in operation.

It is a still further object of this invention to provide a self-starting one-shot bridgewire arc lamp which is simple to manufacture and which, therefore, may be constructed by utilizing mass production techniques.

Itis a still further object of this invention to provide a one-shot bridgewire arc lamp in which the arc is started and sustained by a single power supply, which is inexpensive to manufacture, extremely reliable in operation and can survive severe environmental conditions.

Briefly, the self-starting arc lamp of this invention accomplishes the stated object by utilizing a conductive wire of substantially uniform cross section for bridging the gap between the electrodes. The wire, also referred to as a bridgewire, may be of single or multiple strand construction and has its end portions securely held by spot welding to the electrode portion immediately following the arc end. To achieve good centering of the wire between the points of the opposed electrodes and to hold the end portions securely against shock and vibration, one or more accommodation slots or grooves are cut into the peripheral surface of the conoidal arc end portions of the electrodes so that the bridgewire is axially suspended between the electrode points and firmly secured along the arc end portions.

Further objects and advantages of the present invention will become apparent to those skilled in the ant to which the invention pertains as the ensuing description.

proceeds.

The features of novelty that are considered characteristic of this invention are set forth with particularity in the appended claims. The organization and method of operation of the invention itself will best be understood from the following description when read in connection with the accompanying drawing in which:

FIG. 1 is a view, partly in elevation and partly in cross section with parts broken away, of a self-starting bridgewire arc lamp constructed in accordance with this invention;

FIG. 2 is an enlarged elevational view of the bridgewire and the adjacent electrode portion of the arc lamp shown in FIG. 1;

FIG. 3 is a view taken along lines 33 of FIG. 3;

FIG. 4 is a view taken along lines 44 of FIG. 2;

FIG. 5 is a perspective view of an alternate embodiment of this invention utilizing a multi-strand bridgewire between the electrodes;

FIG. 6 is a perspective view of the bridgewire and electrodes of FIG. 5 illustrating the assembling ope'ra-' tion; and

,. Illustrated arc lamp 10 is of the general class referred to as a short are lamp which may be filled with an inert gas such as Xenon. Such short are lamps usually contain the gas under pressure which, at room temperature, typicallyis from between 3 to atmospheres. At the operating temperature the gas pressure increases typically to about 15 to 40 atmospheres. The reason these lamps are referred to as short arc is because the anode and cathode are separated by a relatively short distance, typical 30 thousands of an inch, to produce an intense arc. In prior are devices the arc is started by the application of a high or firing voltage from a starting power supply which causes, in the order stated, Townsend discharge, glow discharge and finally arc discharge. Once started the arc is maintained by the low voltage power from a sustaining power supply. In the instant invention, the sustaining power supply also starts the arc.

Lamp 10 includes a pressurized envelope 11 which, for structural strength, has a spherical bulb portion 12 constructed of quartz to be transparent to short wave length radiation for well understood reasons. Envelope 11 also includes a pair of axially aligned stem portions 14 and 1 6 on opposite sides of bulb portion 12 and integral therewith.

Mounted in stem portion 14 is an anode 18 of refractory metal, usually swaged tungsten, which has a cylindrical main body 22, a tab portion 20 and a conical arc end portion 21. Tab portion 20 is welded or otherwise conductively connected to a thin conductive ribbon 24 which is sealed into the closed stem portion 14 and which provides a current connection to an external connector 26.

Mounted in stem portion 16 is a cathode 28, made of similar material as that used for anode 18, which likewise has a cylindrical main body 32, a tab portion 34 and a conical arc end portion 3-1. Main body 32 of cathode 28 is usually smaller in diameter than main body 22 of anode 18 as illustrated. As before, tab portion 30 is welded or otherwise conductively connected to thin conductive ribbon 34 which is sealed into the closed end portion of stem 16 and which serves as a current lead thereto. Ribbon 34 is electrically connected to an external connector 36. Power is applied to are lamp 10 by connecting a power supply (not shown) across connectors 26 and 36.

Main bodies 22 and 32 of anode 18 and cathode 28, respectively, are in axial alignment and facing arc end portions 21 and 31 are shaped to define cones whose apexes lie on the common anode and cathode axis. This geometry facilitates the drawing of the arc therebetween, and once formed maintains the are in a definite position and prevents the arc ends from wandering over the electrodes.

Spanning the gap between electrode arc end portions 21 and 31, and in electrical contacts therewith, is a fusible bridgewire which has a substantially uniform cross section along its length. The preferred material of which bridgewire 40 is constructed depends, among other factors, on the particular application of lamp 10, its method of construction and the type of gas in its envelope. Generally speaking, bridgewire 40 may be made of any conductive metal such as iron, copper, tungsten, molybdenum, platinum, tantalum, or the like, the particular metal selected depending on the above named factors.

Incase of arc lamps which require bake-out during their construction, the material selected for bridgewire 40 would be a refractory metal which is capable of withstanding bake-out temperatures which typically range between 1400 to l 800 centigrade. In case of are lamps filled with hydrogen, the preferred bridgewire material is a noble metal such as platinum or a platinum clad metal such as tungsten. For arc lamps which are used in an environment subjecting them to severe shock and vibration, a refractory metal whose ductility remains unimpaired by the high bake-ou-t temperatures is selected and in this connection tantalum has been found to be a preferred material. It is to be understood that materials other than those described herein-above may be used for the construction of bridgewire 40, and that substantially all conductive materials are suitable. The particular conductive material chosen depends primarily on the environmental conditions which the bridgewire has to withstand.

The diameter of bridgewire 40, or more correctly its cross section area, must meet two conflicting requirements. Firstly, it should be as large as possible for ease of manufacture and assembly of the arc lamp and to have minimum electrical resistance. Secondly, it must be sufficiently small to fuse or burn out before the arc lamp is damaged by the initial current flowing therethrough.

The first requirement is obvious since fine wires are difiicult to handle and tear easily. The second requirement is particularly important for ribbon seal lamps because such lamps are easily damaged by excessive currents which burn out the ribbon. The ribbons are typically rectangular in cross section, having a width of 60 to thousandths and a thickness of 0.5 thousandth of an inch. It has been found that ribbons of the cross sectional area just described can withstand a current of about 30 to 40 amperes for a period of about one-tenth of a second with out burnout. This is particularly true where a quartz envelope is utilized which is shrunk upon the whole length of the ribbon and therefore conducts heat away from the ribbon. Accordingly, a cross sectional area of bridgewire 40 which causes the bridgewire to fuse in less than one-tenth of a second upon application of a current of about 30 to 40 amperes meets this requirement. For example, a tantalum wire having a diameter of about 6 thousandths of an inch has been found to meet this requirement. In case of rod seals, where the danger of lamp damage due to starting current flow is much reduced, a wire of greater diameter may be used.

Referring now particularly to FIGURES 2, 3 and 4, bridgewire 40 spans the gap between electrode arc end portions 21 and 31 which is typically 30 thousandths of an inch long. To support bridgewire 40 axially between the apexes of anode 1'8 and cathode 28, each electrode is provided with a substantially radial slot shown at 42 and 44 respectively. Radial slots 42 and 44 have side walls parallel to and symmetric with respect to a radial plane of the electrode and a bottom wall which is curved and which extends from the apex of the electrode arc end to a convenient point on the cylindrical body of the electrode. The width of each slot is substantially the same as the diameter of bridgewire 40. Slots 42 and 44 he on opposite sides of a common diametrical electrode plane so that opposite ends of bridgewire 40 are supported from opposite sides of the common diametrical plane. Of course, slots 42 and 44 need not be exactly opposite to one another as long as they are disposed in such a manner that the bridgewire is axially supported by the opposite slot ends at the electrode apexes against falling out of.

the slots; This requirement is best described by the criteria that the slots must be of a depth and must have a relative orientation suchthat, once the ends of the bridgewire are secured to the electrode bodies, attempted removal of the bridgewire from either slot puts the bridgewire under tension. Further, instead of the deep slots shown, shallow accommodation grooves may be substituted.

In this manner bridgewire 40, upon being inserted into tab 46 for conductive contact with anode 18, and to tab 48 for conductive contact with cathode 28.

In operation, a power supply (not shown) is connected across terminals 26 and 36 causing a current to flow through ribbon 24, cathode 18, bridgewire 40, anode 28 and ribbon 34, in that order. Since bridgewire 40is' highly conductive, the starting current will be high, say between 30 and 40 amps. This starting current will heat up ribbon 24, bridgewire 40 and ribbon 34, the temperature rise of the larger electrodes may be neglected. Since the cross sectional area of bridgewire 40 is carefully selected to burn out before the ribbons, a gap will be formed as a section of the bridgewire is consumed. This, of course, reduces current flow.

Since slots 42 and 44 have approximately the same width as the diameter of bridgewire 40, they are in good thermal contact with the bridgewire and the electrodes act as a heat sink. For this reason, the heat generated in bridgewire 40 by the starting current will be carried away by the electrodes primarily from the portions of the bridgewire immediately adjacent to the electrodes, thereby causing a hot spot at the center portion of the bridgewire. This will cause the center portion of bridgewire 40 to reach its melting point first and to commence fusion and to burn out. As soon as the center portion burns an arc will form which will progressively consume the remaining bridgewire until the arc spans the gaps between the electrodes. As the width of the gap increases, the resistance between the electrodes increases and the current decreases thereby preventing dam-age to ribbons 24 and 34.

It is to be noted that in cases of rod seal arc lamps, in which there is little danger of damage to the arc lamp from high current, the cross sectional area of the bridgewire may be larger to facilitate its handling during manufacture. This invention is applicable to both ribbon seal and rod seal arc lamps. The reason a ribbon seal arc lamp has been selected for illustration is because of the added consideration of the cross sectional area of the bridgewire to prevent damage thereto. In all other respects, the above described configuration is equally applicable to rod seal arc lamps.

Referring now to FIGURES 5, 6 and 7, there is shown an alternate embodiment of a self starting bridgewire arc lamp which incorporates features providing certain manufacturing advantages. An anode 50 having an electrode arc end portion 52 of conical shape, and a cathode 54 having an electrode arc end portion 56 likewise of conical shape are supported within an envelope (not shown) which may be similar in all respects to envelope 12 of FIGURE 1. Are end portions 52 and 56 are provided with wire reception means in the form of diametrically opposed shallow grooves 58 and 60 respectively as shown. Instead of grooves the wire reception means may be formed by slots which bisect the conical end portions and which have side walls substantially parallel to and symmetric with a diametrical plane. Additionally, cathode 54 is provided with a slanted deep slot 62 whose side walls make approximately a 45 angle with the axis of cathode 54.

A bridgewire 70, which is similar in most respects to bridgewire 40 of FIGURE 1 except that it has two strands twisted together, spans the electrode gap. If two-strand bridgewire 70 is utilized with a ribbon seal arc lamp, the combined cross sectional areas of the individual strands are subject to the same limitations set forth above, that is the bridgewire must fuse before the lamp is dam-aged. In case of utilization of a multi-strand bridgewire in rod seal are lamps, the limitation on the combined cross sectional area of the bridgewire is of lesser importance.

Utilization of a twisted multi-strand bridgewire has a number of manufacturing advantages, an important one being that the length of the bridgewire in the gap is adjustable by twisting. It is well known that the length of the electrode gap is subject to rather stringent design specifications. Since the bridgewire should not sag and preferably should be slightly tensioned between the electrodes for greatest survival in a severe environment, utilization of a single strand bridgewire presents a number of manufacturing difliculties. Since the bridgewire ends are spot welded to the electrodes before final assembly, there is little or no adjustment possible. In case of attwo-strand twisted bridgewire, adjustment of the bridgewire length in the gap is provided by twisting the electrodes in opposite directions until the wire is properly tensioned.

In assembling an arc lamp having a twisted bridgewire 70, the center portion of the wire is inserted into slot 62 and the two end portions are welded to the peripheral surface of anode50, either directly as shown or by means of platinum tabs as illustrated in FIGURE 2. Thereafter, anode 50 and cathode 54 are rotated in opposite directions with respect to one another to start twisting the strand and to drawthe two-strand portion into wire reception grooves 58 and 60 respectively or into slots. The elec trodes are then moved to achieve proper gap spacing. Should the portion of bridgewire 70 spanning the gap be loose, rotation of one electrode with respect to the" other will change the length until the bridgewire is properly tensioned. The envelope may "then be sealed in the ordinary manner.

The operation of the arc lamp illustrated in FIGURE 5 is the same in all respects to that of FIGURE 1. As long as wire reception grooves or slots 58 and 60 are substantially of the same width as the diameter of bridgewire 70, good thermal contact between the wire and the electrodes is provided. This causes a hot spot at the center of the gap and therefore burnout will occur at the center of the gap.

There has been described a self-starting bridgewire arc lamp in which the application of power from a sustainingpower supply will cause the bridgewire to fuse and to draw an arc between the electrodes. The bridgew'ne may be of single or multiple strand configuration and made of a material selected in accordance with application, type and construction of the arc lamp.

While the above detailed description has shown, described and pointed out the fundamental novel features of the invention as applied to various embodiments, it will be understood that various omissions and substitutions and changes in the form and details of the deviceillustrated may be made by those skilled in the art, Without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

We claim: 1

1. A self-starting are lamp comprising:

a sealed envelope having disposed therein an anode and a cathode which are spaced apart to define an arc gap therebetween;

connector means coupled to said anode and to said cathode and passing through said sealed envelope for applying electrical operating'power across said anode and cathode; and

a consumable, conductive bridgewire of substantially uniform cross section disposed axially across said are gap and having opposite end portions conductive- 1y coupled to said anode and said cathode respectively, the cross sectional are-a of said bridgewire being selected sufficiently small to fuseupon the application of electrical operating power across said anode and cathode which corresponds to the normal operating power for sustaining an are between said anode and said cathode.

2. A self-starting arc lamp comprising:

a sealed envelope having disposed therein an anode and a cathode which are spaced apart to define an arc gap therebetween;

connector means conductively connected to said anode and said cathode and passing through said sealed envelope for applying electrical operating power across said anode and cathode; and

a consumable conductive bridgewire of substantially uniform cross section disposed axially across said are gap and having opposite end portions conductively coupled to said anode and said cathode respectively, the cross sectional area of said bridgewire being selected to fuse upon the application of electrical operating power, corresponding to the normal operating power for sustaining an are between said anode and cathode, in a period which is less than one second.

3. An arc lamp in accordance with claim 2 in which said bridgewire is made of tantalum.

4. An arc lamp in accordance with claim 3 in which the bridgewire is of substantially circular cross section and has a diameter of less than 0.010 of an inch.

5. In an arc lamp having a pair of spaced apart electrodes defining an arc gap therebetween and disposed within a sealed envelope, and having a substantially constant voltage power supply connected across the electrodes whose output voltage is selected to provide arc sustaining power across the arc gap, the self-starting feature comprising:

a consumable, conductive bridgewire of substantially uniform cross section and having at least one strand of Wire and axially disposed between the arc ends of said electrodes and connected, at opposite ends, to the peripheral surface of said electrodes, said electrodes including peripheral bridgewire reception means having a width for snugly receiving each strand of wire for accommodating the bridgewire portion coextensive with each electrode, said reception means commencing at the tip of the electrode arc ends and terminating short of the point of connection of the bridgewire to the electrodes.

6. An arc lamp in accordance with claim 5 in which the bridgewire comprises a single strand of wire and in which the bridgewire reception means in one electrode is diametrically opposed to the reception means in the other electrode.

7. An arc lamp in accordance with claim 5 in which the bridgewire comprises a single strand of wire, and in which one of said electrodes further includes a bridgewire retaining means in the portion adjacent the arc end portion for retaining the center portion of said bridgewire, said last mentioned electrode having its bridgewire reception means diametrically opposed for accommodating the bridgewire portions on either side of said retaining means, the portions of said bridgewire extending past said last mentioned reception means being twisted across said gap, said other electrode having its bridgewire reception means diametrically opposed for accommodating the bridgewire portions coextensive with said other electrode, the ends of said bridgewire extending past said last mentioned reception means being connected to said last mentioned electrode.

8. In an arc lamp having a pair of spaced apart electrodes defining an arc gap therebetween and disposed within a sealed envelope, and having a substantially constant voltage power supply connected across the electrodes whose output voltage is selected to provide arc sustaining power across the arc gap, the self stanting feature comprising:

a consumable, conductive bridgewire of substantially uniform cross section axially disposed between the arc ends of said electrodes and having opposite ends connected to said electrodes, the cross sectional area and material of said bridgewire being selected so that said bridgewire is consumed in a period which is less than 1 second upon application of power from the power supply.

'9. A self-starting arc lamp for use with a power supply delivering substantially constant power, said are lamp comprising:

a pressurized envelope;

an anode of substantially cylindrical configuration and terminating in a conoidal end portion defining its arc end, said anode including a bridgewire accommodation means extending along the conoidal surface from its apex to the cylindrical portion;

a cathode of substantially cylindrical configuration and terminating in a conoidal end portion defining its arc end, said cathode including a bridgewire accommodation means extending along the conoidal surface from its apex to the cylindrical portion;

means for mounting said anode and said cathode within said envelope to define a gap of predetermined length therebetween, the anode and cathode configuration and gap length being selected for optimum operation conditions for sustaining an arc across said gap by means of the power supply;

connector means passing through said envelope for connecting said anode and cathode to the power supply; and

a bridgewire of substantially uniform cross section axially disposed in said gap, opposite end portions of said bridgewire being respectively received by the accommodation means in the conical end portions of said anode and cathode and being conduotively connected to the peripheral surface of the cylindrical portion of said anode and cathode.

10. A self-starting arc lamp in accordance with claim 9 in which said accommodation means are radial slots having a Width to snugly receive said bridgewire and in which said bridgewire has a diameter to fuse in less than one second after the application of power from said power supply.

11. A self-starting arc lamp for use with a power supply delivering substantially constant arc sustaining power, said are lamp comprising:

a pressurized envelope;

a pair of electrodes each having a main body portion and a substantially conical end portion atone end thereof, each electrode including a bridgewire accommodation means in the conical end portion terminating at the apex;

means for mounting said electrodes Within said enve lope to define a gap of predetermined length between opposed apexes, the length of said gap'and the configuration of said electrodes being selected for optimum arc sustaining conditions for the power supply and said electrodes being oriented such that said accommodation means are diametrically opposed;

means passing through said envelope for connecting said electrodes to said power supply; and

a consumable bridgewire of substantially uniform cross section along its length disposed axially across said gap, opposite end portions of said bridgewire being received by said accommodation means and being conductively connected to the body portions of said electrodes, said accommodation means being dimensioned to provide contact with diametrically opposed sides of said bridgewire, said bridgewire being made of a material and being dimensioned to fuse in less than 1 second upon application of arc sustaining power from the power supply.

12. A self-starting arc lamp in accordance with claim 9 in which said bridgewire is made of tantalum and has a diameter which is less than 0.010 inch.

References Cited by the Examiner UNITED STATES PATENTS 412,599 10/18'89 Coffin 219-400 1,898,060 2/1933 Noble 219- X 2,215,300 9/ 1940 Ryde 313-184 2,391,611 12/1945 Back 315--241 X 2,472,923 6/1949 Schmerber 219146 X RICHARD M. WOOD, Primary Examiner.

JOSEPH V. TR-UHE, Examiner. 

1. A SELF-STARTING ARC LAMP COMPRISING: A SEALED ENVELOPE HAVING DISPOSED THEREIN AN ANODE AND A CATHODE WHICH ARE SPACED APART TO DEFINE AN ARC GAP THEREBETWEEN; CONNECTOR MEANS COUPLED TO SAID ANODE AND TO SAID CATHODE AND PASSING THROUGH SAID SEALED ENVELOPE FOR APPLYING ELECTRICAL OPERATING POWER ACROSS SAID ANODE AND CATHODE; AND A CONSUMABLE, CONDUCTIVE BRIDGEWIRE OF SUBSTANTIALLY UNIFORM CROSS SECTION DISPOSED AXIALLY ACROSS SAID ARC GAP AND HAVING OPPOSITE END PORTIONS CONDUCTIVELY COUPLED TO SAID ANODE AND SAID CATHODE RESPECTIVELY, THE CROSS SECTIONAL AREA OF SAID BRIDGEWIRE BEING SELECTED SUFFICIENTLY SMALL TO FUSE UPON THE APPLICATION OF ELECTRICAL OPERATING POWER ACROSS SAID ANODE AND CATHODE WHICH CORRESPONDS TO THE NORMAL OPERATING POWER FOR SUSTAINING AN ARC BETWEEN SAID ANODE AND SAID CATHODE. 